This summer, I purchased a '99 Taurus. It was running great with a few minor problems here and there. However, as the weather has gotten colder, it has developed an issue with getting up to the proper running temperature (it is running very close to cold…closer when it’s colder outside). Last night it was down in the 30s outside, and I drove for two hours at 55-65 mph, and the temperature gauge was still reading cold. I have had the thermostat replaced, and the heater core has been flushed multiple times. The mechanic that has looked at it is not sure what is wrong with it. I’m at a complete loss as to where to go from here. What should I do?

My first thought is just that there is something wrong with the thermostat since that is exactly what these symptoms point to. Strange things happen during working on cars. Was the cooling system flushed when the thermostat was replaced/heater core flushed? For a manual flushing the thermostat is removed. Perhaps someone literally just forgot to put it back in. Another possibility is that the new thermostat was bad off the shelf. It happens.

If the car’s temp gauge was going to normal I’d say to keep looking at a heater core problem. But since the temp gauge is staying low this really just points to a stuck open thermostat.

Its possible that the car is actually getting up to temperature then. Has your shop checked the actual coolant temperature after the car has been running a while? And did they check the coolant temperature sensor? Someone needs to do those things to find out if the coolant is getting hot or not. Someone should also find out whether or not the heater hoses are getting hot - both sides - to see if the heater core is actually flowing.

Thermostats can be had in different temperature ranges. A typical OEM thermostat would be a 180-200 degree or so. But one can get lower temp thermostats. Was the new thermostat OEM?

Maybe you have 2 completely different problems - a bad coolant temp sensor and a stuck blend door in your HVAC ducts. This is really a stretch - especially b/c what you describe sounds just like a stuck open thermostat - but if the thermostat has been checked and double checked then there are other things to look at.

Not sure if they did this or not, but I am sure that the thermostat was OEM as I purchased that. So would the coolant temperature sensor cause the gauge in the car to show that it’s running cold? Thanks so much for the ideas!